import numpy as np

N = int(input())
A = tuple(map(int, input().split()))
INF = 10000

c, h = 0, 0  # 肉食, 草食
dp = np.full(N + 1, INF)
dp[0] = 0
for i in range(N + 1):
    if i == N or A[i] == 2:
        if c == h == 0:
            continue
        new_dp = dp + c  # 草食に揃える
        new_dp[c + h:] = np.minimum(new_dp[c + h:], dp[:-c - h] + h)  # 肉食に揃える
        dp = new_dp
        c, h = 0, 0
    elif A[i] == 0:
        c += 1
    elif A[i] == 1:
        h += 1
res = dp[A.count(0)]
if res >= INF:
    print(-1)
else:
    print(res // 2)